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United States Patent |
6,171,254
|
Skelton
|
January 9, 2001
|
Control for automatic blood pressure monitor
Abstract
A blood pressure monitoring system for automatic unattended operation uses
curve fitting techniques determined during an initial inflation period to
determine cuff size. Based upon cuff size, the number of important
pre-determined operating parameters are determined for use in controlling
the remaining blood pressure reading operations. The automatic blood
pressure monitor according to the present invention offers a simplified,
cost effective construction utilizing a single pump, a single valve and a
single valve orifice.
Inventors:
|
Skelton; Brian J. (Lake Zurich, IL)
|
Assignee:
|
Medical Research Laboratories, Inc. (Buffalo Grove, IL)
|
Appl. No.:
|
258980 |
Filed:
|
February 26, 1999 |
Current U.S. Class: |
600/490; 600/485; 600/493 |
Intern'l Class: |
A61B 005/02 |
Field of Search: |
600/485-500
|
References Cited
U.S. Patent Documents
3585987 | Jun., 1971 | Svensson | 128/2.
|
3699945 | Oct., 1972 | Hanafin | 128/2.
|
3744490 | Jul., 1973 | Fernandez | 128/2.
|
4109646 | Aug., 1978 | Keller | 128/2.
|
4116230 | Sep., 1978 | Gorelick | 128/2.
|
4501280 | Feb., 1985 | Hood, Jr. | 128/677.
|
4572205 | Feb., 1986 | Sjonell | 128/686.
|
4768518 | Sep., 1988 | Peltonen | 600/490.
|
4924873 | May., 1990 | Sorensen | 128/677.
|
4969466 | Nov., 1990 | Brooks | 120/681.
|
5003981 | Apr., 1991 | Kankkunen et al. | 128/677.
|
5022403 | Jun., 1991 | LaViola | 128/680.
|
5060654 | Oct., 1991 | Malkamaki et al. | 128/686.
|
5069219 | Dec., 1991 | Knoblich | 128/679.
|
5172697 | Dec., 1992 | Koven et al. | 128/679.
|
5240008 | Aug., 1993 | Newell | 128/685.
|
5243991 | Sep., 1993 | Marks | 128/686.
|
5301676 | Apr., 1994 | Rantala et al. | 128/686.
|
5447160 | Sep., 1995 | Kankkunen et al. | 128/677.
|
5746213 | May., 1998 | Marks | 128/686.
|
Primary Examiner: O'Connor; Cary
Assistant Examiner: Carter; Ryan
Attorney, Agent or Firm: Fitch, Even, Tabin & Flannery
Claims
What is claimed is:
1. An automatic, non-invasive blood pressure measuring device of the type
which detects blood pressure pulses in a patient's appendage, comprising:
a cuff for constricting blood flow in the patient's appendage;
a pump connected to the cuff for inflation thereof in response to a pump
control signal;
valve means connected to the cuff for deflation thereof in response to a
valve control signal;
a pressure sensing means connected to the cuff to sense pressure in the
cuff and to send a pressure signal indicating a pressure-time
characteristic of said cuff in response thereto;
microprocessor means connected to said pump, said valve means and said
pressure sensing means, including means to observe the initial
pressure-time characteristic of said cuff during an observed inflation
period in which the pressure of the cuff is increased to a level less than
a target pressure needed to take a blood pressure reading; and
said microprocessor means including means for determining the cuff size by
comparing the initial pressure-time characteristic of said cuff with a set
of stored pressure-time characteristics of cuffs of known sizes, and means
for determining, in response to said cuff size determination, a cuff
inflation rate, a cuff deflation rate and an initial target pressure, said
microprocessor means sending control signals to said pump to inflate said
cuff to said target pressure according to said cuff inflation rate, and to
deflate said cuff at said deflation rate, using said at least one
deflation pressure drop step.
2. The device of claim 1 wherein said pressure sensing means is employed to
detect the blood pressure pulses in a patient's appendage.
3. The device of claim 1 wherein said microprocessor means sends control
signals to said pump to inflate said cuff to approximately said target
pressure at a substantially constant flow rate.
4. The device of claim 1 wherein said microprocessor means sends control
signals to said pump to deflate said cuff at a substantially constant
deflation rate.
5. The device of claim 1 wherein said microprocessor means sends control
signals to said pump to reduce the inflation rate immediately before
sending control signals to said valve to deflate said cuff.
6. The device of claim 1 wherein said set of stored pressure-time
characteristics of cuffs of known sizes comprises a table of a plurality
of discrete pressure and time values, said plurality corresponding to the
number of cuff sizes.
7. The device of claim 1 wherein, during said observed inflation period,
inflation is carried out for a preselected period of time.
8. The device of claim 7 wherein, during said observed inflation period,
inflation is carried out at a substantially constant flow rate.
9. The device of claim 1 further comprising an acoustical detection means
which is employed to detect the blood pressure pulses in a patient's
appendage.
10. A method for the non-invasive automatic measuring of blood pressure of
a patient by detecting blood pressure pulses in a patient's appendage,
using a blood pressure cuff of unknown size to selectively restrict blood
flow in the patient's appendage, comprising the steps of:
providing a pump connected to said cuff for inflation thereof in response
to a pump control signal;
providing a valve means connected to said cuff for deflation thereof in
response to a valve control signal;
providing pressure sensing means connected to said cuff to sense pressure
in said cuff and to send a pressure signal indicating a pressure-time
characteristic of said cuff in response thereto;
providing a microprocessor means coupled to said pump, said valve means and
said pressure sensing means;
sending a pump control signal to said pump to inflate said cuff during an
observation period to a pressure below that required to take a blood
pressure reading, and to observe the pressure-time characteristic during
said observation period;
comparing the pressure-time characteristic obtained during said observation
period to a set of pressure-time characteristics of cuffs of known sizes
so as to determine the cuff size employed;
determining, in response to the determination of the cuff size, a target
pressure and an inflation rate to inflate the cuff to approximately the
target pressure needed to take a blood pressure reading, the deflation
rate of the cuff during a blood pressure reading period;
inflating said cuff according to said inflation rate; and
deflating said cuff according to said deflation rate.
11. The method of claim 10 wherein blood pressure readings are taken in
between said deflation pressure drops.
12. The method of claim 10 wherein at least two blood pressure pulses are
observed between consecutive deflation pressure drops.
13. The method of claim 10 wherein said cuff is inflated at a substantially
constant inflation rate.
14. The method of claim 10 wherein said cuff is deflated at a substantially
constant deflation rate.
15. The method of claim 10 wherein the inflation rate is reduced
immediately before deflating said cuff.
16. The method of claim 10 wherein said set of pressure-time
characteristics of cuffs of known sizes is stored in a table of a
plurality of discrete pressure and time values.
17. The method of claim 10 wherein said observed inflation period is
carried out for a preselected period of time.
18. The method of claim 10 wherein said observed inflation period is
carried out at a substantially constant flow rate.
19. The method of claim 10 wherein said pressure sensing means is employed
to detect the blood pressure pulses in a patient's appendage.
20. The method of claim 10 wherein, in said deflating step, said cuff is
deflated using a valve duty cycle determined in response to said cuff size
determination.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the real time monitoring of a patient's
blood pressure and in particular to the taking of continuous automatic
blood pressure readings.
2. Description of the Related Art
In working with a large number of different automatic blood pressure
reading systems, it has been recognized that the deployment of the blood
pressure cuff must be carefully considered in order to achieve accuracy in
the blood pressure readings taken. It has been observed, for example, that
the width of the blood pressure cuff (taken in the direction along the
length of the patient's arm) must be maintained within certain ranges in
order to prevent erroneous blood pressure readings.
Most of the blood pressure cuffs in use today take the form of a
double-ended, elongated strip which is wrapped about as patient's limb
with ends of the blood pressure cuff partly overlapping. As a minimal
requirement, the amount of overlap must be sufficient to allow proper
self-attachment of the strip ends so as to free an operator to perform
other tasks, such as operating monitoring equipment. Recently, attention
has been paid to the amount of overlap of the blood pressure cuff ends,
with the appreciation that errors in overwrap, either too large or too
small, even if satisfactory to allow blood pressure readings to be taken,
result in an unwanted shift of those readings.
In addition to variations encountered in applying a blood pressure cuff to
a patient's limb, a variation of blood pressure readings also arises from
the fact that, as a practical matter, there are a relatively large number
of different size cuffs by manufacturers of blood pressure reading
equipment. For example, systems having nine or more differently sized
blood pressure cuffs are not uncommon. Cuff sizes typically include a
smallest size blood pressure cuff for neonatal patients and a largest
blood pressure cuff size for adult thigh readings. Some blood pressure
reading equipment requires the user to specify the cuff size by a special
purpose input, such as a special, identifying switch or some other
pre-defined selection means. Other systems require that special pneumatic
fittings be employed to provide a self-identification of the size of the
blood pressure cuff with which the fitting is associated. Accordingly,
some type of keying system between the blood pressure cuff and the
associated pneumatic circuitry is employed. The complexities in taking
blood pressure readings is growing at a time when increasing demands are
being made on care givers and other personnel charged with the
responsibility of taking blood pressure readings. The need for an
improved, automatic blood pressure reading system still exists.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a system and method for
the automatic, continuous reading of blood pressure.
Another object of the present invention is to provide a system and method
of the above-described type which are suitable for use with a plurality of
differently sized blood pressure cuffs and which automatically adapt for
the accurate use of such cuffs.
A further object of the present invention is to provide a system and method
for the automatic reading of blood pressure in which blood pressure cuff
size is automatically determined at the initial phase of a blood pressure
reading, before the actual blood pressure reading commences, allowing for
the calculation of several parameters important to the rapid, comfortable
and safe reading of a patient's blood pressure.
These and other objects of the present invention are provided in an
automatic, non-invasive blood pressure measuring device of the type which
detects blood pressure pulses in a patient's appendage, comprising:
a cuff for constricting blood flow in the patient's appendage;
a pump connected to the cuff for inflation thereof in response to a pump
control signal;
valve means connected to the cuff for deflation thereof in response to a
valve control signal;
a pressure sensing means connected to the cuff to sense pressure in the
cuff and to send a pressure signal in response thereto;
microprocessor means connected to said pump, said valve means and said
pressure sensing means, including means to observe the initial
pressure-time characteristics of said cuff during an observed inflation
period in which the pressure of the cuff is increased to a level less than
a target pressure needed to take a blood pressure reading; and
said microprocessor means including means for determining the cuff size by
comparing the initial pressure-time characteristic of said cuff with
stored pressure-time characteristics of cuffs of known sizes, and means
for determining, in response to said cuff size determination, a cuff
inflation rate, a cuff deflation rate and at least one deflation pressure
drop step size, said microprocessor means sending control signals to said
pump to inflate said cuff to said target pressure according to said cuff
inflation rate, and to deflate said cuff at said deflation rate, using
said at least one deflation pressure drop step.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of an automatic blood pressure monitor according
to the principles of the present invention;
FIG. 2 shows an inflation profile with operation according to principles of
the present invention;
FIG. 3 is a graph showing the pump flow rate associated with the operating
curve of FIG. 2; and
FIG. 4 shows an initial portion of the operating curve, taken on an
enlarged scale.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Turning now to the drawings, and initially to FIG. 1, the present invention
is directed to an automatic blood pressure monitor 10 with automatic cuff
size determination and cuff pressure control. A cuff 12 of conventional
construction is coupled through a hose, piping or other conduit means to a
pneumatic control system, including a pump 16, a valve 18 and a pressure
transducer 20. The cuff 12 is wrapped about the patient's arm and operated
so as to apply varying amounts of pressure sufficient to selectively
occlude and release blood flow through the patient's brachial artery.
Pump 16 is preferably of the positive displacement type operated under
control of an electronic system 24. Most preferably, the pump is
controlled by duty-cycling the driver circuitry of the pump. The driver
portion of system 24 is coupled by control circuit wiring 26 to a
microprocessor 30. Microprocessor 30 is of a conventional type issuing
control instructions, e.g., in the form of a pulse train, to the pump
driver of system 24. The presence or absence of pulses in the pulse train
control the duty cycle of the pump, which in turn directly controls the
pump output, i.e., the inflation rate and inflation volume of cuff 12.
The valve 18 is pneumatically coupled to cuff 12 and provides selective
venting or deflation of the cuff in a controlled manner, preferably by
duty-cycling the driver circuitry 34 of the valve. Valve driver 34 is
coupled by control circuitry 36 to microprocessor 30. The valve 18 is
preferably of the on/off control type (as opposed to more costly
proportional valves). Depending upon the duty cycle control signal
transmitted through conductor 36 to valve control circuitry 34, the valve
is held closed or open with a number of different deflation rates.
A pressure transducer 20 monitors the pressure of cuff 12 and sends an
electrical output signal indicating the pressure, via conductor 40 which
couples the pressure transducer 20 to an analog/digital converter 33. The
analog/digital converter 44 is in turn coupled to microprocessor 30 by
conductor 46. Preferably, the driver circuitry 24 of pump 16 operates
under closed loop control implemented by microprocessor 30. Similarly, the
valve driver circuitry 34 of valve 18 undergoes closed loop operation
under control of microprocessor 30. Although principles of the present
invention may be readily employed with pneumatic control systems having
multiple pumps, multiple valves and/or multiple orifices, present
invention is particularly advantageous in providing heretofore
unattainable control with simple, low-cost components, including a single
pump, a single valve and a single valve orifice arrangement.
In the preferred embodiment, the monitoring system uses an oscillometric
method of determining blood pressure, sensing pulses with pressure
transducer 20. If desired, as an alternative, an acoustic, ultrasonic or
strain gage transducer 52 could be located in the vicinity of cuff 12 for
audibly monitoring blood flow in the brachial artery. The alternative
transducer is shown coupled to conventional pulse discriminator circuitry
54 which could also be implemented, for example, in microprocessor 30.
Together, the transducer and pulse discriminator circuitry, either
standing alone or incorporated in microprocessor 30, detect the presence
of blood flow in the patient's appendage, monitor the number of
heart-induced pulses in the arterial blood flow and measure the relative
amplitudes of those arterial pressure pulses.
Typically, the blood pressure cuff 12 is initially inflated to a
suprasystolic pressure level at which blood flow is cut off in the
patient's limb, herein the brachial artery. As an alternative to a
complete cessation of blood flow in the brachial artery, pressure can be
increased in cuff 12 so as to apply sufficient pressure to impede blood
flow in the brachial artery to a point where the pulse beat is either
substantially reduced or can no longer be detected. Thereafter, pressure
applied to the brachial artery by the cuff 12 is reduced by relaxing
pressure in cuff 12 in a controlled manner until the first very weak pulse
is detected, and this pressure is immediately related to a pressure above
the patient's systolic blood pressure level. As the pressure in cuff 12 is
reduced, the pressure is continuously detected by transducer 20 and
monitored by microprocessor 30.
After the first faint pulses are reliably detected, cuff pressure is
reduced by a controlled amount and held at the reduced level for a defined
period of time, long enough to acquire additional pulse information. Most
preferably, each "pressure hold" step is sustained long enough to reliably
detect two adequately discerned blood pressure pulses. Eventually, with a
sufficient number of cuff pressure reductions having been carried out, the
amplitude of the blood pressure pulses is typically observed to rise to a
maximum value and then fall to a point where blood pressure pulses can no
longer be detected, an operating point below the patient's diastolic
pressure reading. As will be seen herein, the present invention affords a
number of significant advantages in obtaining blood pressure readings in
as short an operating time as is practical.
Briefly, the present invention operates early on so as to identify as
quickly as possible the size of the blood pressure cuff in sufficient time
so as to allow calculation of a number of important control parameters and
to thereafter control a substantial portion (and preferably the major
portion) of the cuff's inflation period (i.e., the time during which the
cuff is brought to a carefully defined patient-specific suprasystolic
pressure level, which is approximately the maximum pressure experienced by
the cuff.
The required sequence of blood pressure readings occurs at points located
below suprasystolic pressures, and taken after the inflation period, when
the cuff is continuously deflated until a sub-diastolic pressure level is
attained. Thereafter, if additional blood pressure pulse information is
desired for the same patient (due, for example, to artifacts caused by
motion), pressure may be increased to a controlled supradiastolic level.
If a complete repetition of the blood pressure analysis is desired,
pressure is elevated once more to a suprasystolic pressure level to enable
a repeated observation of the patient.
It is important that cuff 12 be inflated as quickly as possible so as to
allow the actual blood pressure measurements to be taken as quickly as
possible. However, it has been found that patients react with alarm to
high inflation rates, particularly those carried out under automatic
control of an unattended machine. This could result in alterations of a
patient's vital signs or induce motion artifacts by agitating the patient.
The maximum pressure level is particularly important for neonatal and
other relatively young patients where a risk of injury may be present if
cuff pressures are allowed to assume elevated levels. Control of neonatal
blood pressure cuffs has traditionally proven to be unusually difficult
because of the smaller volume capacities of the blood pressure cuff used
on relatively young patients. The present invention offers improved
protection while allowing very rapid determination of blood pressure cuff
size, one which can be taken using relatively inexpensive components and
in such a rapid manner that pressure levels even for relatively small
neonatal cuff sizes are well below acceptable elevated pressure levels.
Once the determination of cuff size has been made, the present invention
determines a number of important operating parameters, including the
"rate" of inflation over the inflation rate period (preferably the
.DELTA.p and .DELTA.t values from beginning to end of the inflation rate
period), the target pressure, the rate of deflation needed to secure data
about the systolic and diastolic blood pressure levels, and the overshoot
control employed in step-wise deflation, i.e., pressure reduction, over
the time period that blood pressure readings are taken.
Turning now to FIGS. 1-3, operation of the blood pressure monitor begins
with cuff 12 substantially deflated, indicated by pressure P.sub.0 in FIG.
2. As will be seen herein, the blood pressure reading cycle is begun
later, approximately at time t.sub.R after an elevated, substantially
maximum target pressure P.sub.3 is reached. As indicated by the operating
curve in FIG. 2, the pressure in the cuff must be substantially increased
beyond the initial pressure P.sub.0 and a substantial amount of time
indicated by the interval between t.sub.0 and t.sub.3 is needed to fully
inflate the cuff. During the time required to inflate cuff 12, a brief
initial inflation period (t.sub.0 to t.sub.1 is defined, and data
characteristic of the cuff is accumulated and analyzed. Based upon the
results, several important factors are calculated at time t.sub.1, in time
to set pump 16 and valve 18 for the remainder of the operation.
As a first step, between time t.sub.0 and t.sub.1, herein the initial
inflation period, cuff 12 is inflated to a relatively low pressure level,
preferably a small fraction of the operating pressure P.sub.3. As
graphically indicated in FIG. 2, this portion of the operating curve
designated C.sub.1 is non-linear and, the curve shape has been found to be
characteristic of the size of the cuff being inflated. During the initial
inflation period, the cuff may be inflated in a number of different ways.
However, the cuff is preferably inflated with a constant flow rate for a
pre-defined period of time. That is, the time interval of the initial
inflation from t.sub.0 to t.sub.1 is preferably fixed as part of the
program control loaded into microprocessor 30. In the preferred
embodiment, the initial inflation period is set so as to assure that, for
the smallest blood pressure cuff possible (usually neonatal size) the
final pressure at a constant flow rate is well below the appropriate
patient-specific maximum operating pressure (i.e., approximately P.sub.3
in FIG. 2). At the end of the initial inflation period, pressure is
elevated to level P.sub.1 and the time interval has allowed a pressure
difference of P.sub.1 -P.sub.0, herein .DELTA.P. It is preferred that the
characteristic shape of the initial inflation curve C.sub.1 is calculated
or otherwise determined immediately at time t.sub.1 by microprocessor 30,
based upon readings of pressures sensed by transducer 20 and converted
into digital form by converter 44.
Referring to FIG. 3, the initial rate of flow is constant throughout the
initial inflation period. In order to obtain as rapid a processing time as
possible, the rate of flow changes indicated by FIG. 3 are carried out in
a step-wise manner, although sloped or curved flow rate changes may also
be employed.
Referring briefly to FIG. 4, a family of characteristic curves for three
different blood pressure cuff sizes is shown. For example, for a "size 1"
cuff, the smallest cuff size shown, the inflation curves lie between the
ordinate and curve S1.sub.max. The characteristic curves for the next
largest cuff size lie between boundaries S2.sub.min and S2.sub.max. The
next largest cuff size is associated with an operating region beginning
with boundary S3.sub.min and extending to the right, beyond the area shown
in FIG. 4.
As mentioned, a relatively large number of cuff sizes is found in modern
commercial blood pressure reading systems. For the system shown in the
preferred embodiment, nine different cuff sizes are assumed. FIG. 4
depicts characteristic operating curves for the three smallest cuff sizes.
It has been found that, due to manufacturing tolerances as well as
variations in the conformance of the materials employed, a single
well-defined characteristic curve is not observed for practical blood
pressure cuffs. Rather, as is indicated in FIG. 4, the characteristic
curve for a plurality of the same size blood pressure cuffs falls within a
range, lying between minimum and maximum limits.
Although more precise recognition schemes can be employed, it has been
found expedient for rapid, real time control to pre-define characteristic
curves CC.sub.1 and CC.sub.2 as shown in FIG. 4 lying within the overlap
regions and most preferably at the maximum observed limits for each
particular size. As can be seen in FIG. 4, the first characteristic curve
CC.sub.1 is located slightly to the left of S1.sub.max, between S2.sub.min
and S1.sub.max. Similarly, characteristic curve CC.sub.2 is located
slightly to the left of curve S2.sub.max, lying between curves S3.sub.min
and S2.sub.max. For the purpose of determining control parameters, it
should be understood that other curves can be employed which are not
related to a specific cuff size. For example, the number of characteristic
curves for the family of adult size cuffs can be reduced in number, since
it has been found that certain control parameters for certain grouped cuff
sizes (neonatal, infant, adult) can be shared for several different cuff
sizes within the same group.
For nine different cuff sizes in the blood pressure monitor of interest,
nine characteristic curves will be predetermined and stored in
microprocessor 30. As cuff pressure data is taken in the initial inflation
period t.sub.0 to t.sub.1, curve data represented as a solid continuous
curve portion C.sub.1 is accumulated in microprocessor 30 and is compared
against the pre-defined characteristic curves. The closest curve fit lying
immediately to the right of the observed curve portion indicates the
determined cuff size. In practice, cuff size determinations can be made
very quickly without substantial delay, at time t.sub.1. A series of
formulas or look-up tables are then employed by microprocessor 30 to
determine a number of important operating parameters which control system
operation beyond time t.sub.1. An example will be given below.
Before proceeding with a further discussion of operating parameters
determined by the present invention, it should be understood that
relatively inexpensive equipment can be employed to acquire and interpret
enough pressure data points between times t.sub.0 and t.sub.1 to form a
substantially solid curved portion as indicated in FIG. 2. It has been
found sufficient in practicing the present invention to forego expensive
computer control equipment and to rely instead on data collected as a
series of spaced apart operating points. In its simplest form, the present
invention looks at the pressure difference over the initial inflation
period and scans a table of characteristic values to determine blood
pressure cuff size. An adjustment must be made, however, for different
initial pressure levels P.sub.0. Other types of "curve fitting" can be
employed using "least squares fit" and other known techniques.
Once the blood pressure cuff size is determined, a number of operating
parameters are determined and are loaded by the control program of
microprocessor 30. One parameter determined is the inflation rate between
time t.sub.1 and time t.sub.2, represented by the step increase at time
t.sub.1 in FIG. 3. It is generally preferred that the operating curve
portion C.sub.2 during this time period be substantially linear in shape,
although other shaped inflation curves could be employed as well.
Preferably, the slope of curve portion C.sub.2 is pre-defined in a look-up
table where other data, based upon observed patient response for the
particular size blood pressure cuff, is stored.
It is desirable to shorten the inflation time period t.sub.1 -t.sub.2 as
much as possible. However, excessive rates of inflation are known to
startle patients if the inflation is perceived as being near
instantaneous, or if the rate of rapid constriction of the patient's
appendage is perceived as being surprisingly steep. All of these factors
tend to alarm certain patients, with expected physiological reactions
resulting. Accordingly, the stored inflation rate values for curve portion
C.sub.2 are a trade-off between speed and unintended patient response.
With the present invention, the differing slope values for differing cuff
sizes can be tailored for optimal results.
Referring again to FIGS. 2 and 3, another important parameter determined by
cuff size is the maximum operating pressure needed for blood pressure
readings, identified as pressure P.sub.3 in FIG. 2. In an effort to avoid
expensive inflation equipment with more elaborate overshoot control, and
to allow the use of single hose cuffs, the automatic control provided by
the present invention provides an over-shoot control time period t.sub.2
-t.sub.3 during which the rate of inflation is reduced a small amount as
indicated by the step drop at time t.sub.2 in FIG. 3. As can be seen in
FIG. 2, the attendant pressure-time response of the cuff is non-linear
but, with prior testing of known cuffs, the time interval of the
over-shoot control period can be accurately determined and stored as a
control parameter in a cuff-size related look-up table accessible by
microprocessor 30. As can be seen in FIG. 2, the pressure reaches an
absolute maximum P.sub.x during the over-shoot control period although,
with sufficient passage of time the pressure level is stabilized at time
t.sub.3 in preparation for the beginning of a blood pressure reading
cycle.
At time t.sub.R the blood pressure reading cycle is initiated along with
deflation of the blood pressure cuff. According to the present invention,
it is preferred that deflation during the blood pressure reading, i.e.,
between times t.sub.3 and t.sub.4, have a constant rate of pressure
change, that is, a linear dP/dt characteristic shape. The pressure level
P.sub.3 is defined at the beginning of the blood pressure reading cycle
and its value is determined based upon the blood pressure cuff size.
Preferably, the beginning reading pressure P.sub.3 is obtained by
consulting a look-up table stored within microprocessor 30. Generally, the
pressure level P.sub.3, although different for each cuff size or perhaps
cuff group (i.e., neonatal, infant, adult), is associated with a
suprasystolic level for the appendage involved. As indicated in FIGS. 2
and 3, it is preferred that pressurization of the cuff is terminated at
approximately time t.sub.3 and thereafter the cuff is deflated by
operation of valve 18 under control of microprocessor 30.
As mentioned, at approximately time t.sub.1 the observed curve data is
compared against stored values to determine the blood pressure cuff size.
This result is used to determine a number of important operating
parameters and the first parameter needed is the inflation rate or
inflation curve shape between times t.sub.1 -t.sub.2. As a practical
matter, the initial target pressure P.sub.3 is also determined about time
t.sub.1, along with the deflation rate between times t.sub.3 and t.sub.4,
as well as the individual overshoot controls and valve duty cycles for
deflation pressure drops indicated in the stair step curve portion
C.sub.4. If desired, the determination of the maximum pressure can be
delayed until a time shortly prior to t.sub.2, the end point of the
over-shoot control period t.sub.3 can be delayed until a time after
t.sub.2 and the deflation rate and step size can be delayed until time
t.sub.3, if desired. However, it has been found expedient to perform all
necessary parameter determinations approximately at time t.sub.1 and this
is found to be readily achievable using modestly priced components.
As will be apparent, a wide variety of step configurations can be employed
to accommodate the same deflation rate slope. It is preferred in
practicing the present invention that the deflation control parameters be
pre-defined for each cuff size and stored in memory, available to
microprocessor 30. Upon identification of the cuff size, the number of
steps defined either by the pressure drop of each step or the time period
between steps is then used to control, preferably in a duty-cycle fashion,
the driver 34 of valve 18 to achieve the performance desired.
Only a few deflation steps are shown in FIG. 2 for purposes of
illustration. It is preferred in practicing the present invention that the
time period between adjacent pressure drop steps be chosen to allow for
artifact and motion rejection for the data collected in the appendage
being monitored. In the example given, the brachial artery is monitored,
and the pressure P.sub.3 applied to the patient's upper arm is high enough
to assure either that blood flow is substantially occluded in the brachial
artery or is otherwise reduced as required. Each step during deflation
allows for matching arterial pressure pulses based upon pre-determined
data for patients monitored by the particular cuff size employed. The
blood pressure pulses are preferably monitored by the pressure transducer
20, but, as mentioned, may also be monitored by microphone 52 and a
conventional audio processor 54, such as one of the Korotkoff pulse
determination type, doppler (ultrasonic) or tonometry (strain gage array)
techniques.
Over the evaluation period t.sub.3 -t.sub.4, after a certain number of
deflation steps are carried out, blood pressure pulses will be detected
and the history of the blood pressure pulse data will be stored in
micro-processor 30 for future reference.
Throughout the deflation period, the amplitude of the blood pressure pulses
will change over time, typically rising to a maximum value at the mean
arterial pressure, thereafter falling to a minimum, final value toward the
end of the blood pressure reading, at a time before time t.sub.4. The
number of steps during the deflation period t.sub.3 -t.sub.4 are chosen to
allow accurate detection of the systolic, diastolic and MAP values, while
assuring that at least two matching pressure pulses are observed for each
step interval. If desired, the blood pressure drop at each step can be
held constant throughout the deflation period or can be varied throughout
the deflation period according to pre-defined values stored in
microprocessor 30 for the particular blood pressure cuff size. In any
event, the ending point t.sub.4 of the blood pressure reading cycle is
chosen to be substantially beyond the time when the final detectable blood
pressure pulse is detected.
It has been found desirable at time t.sub.4 to reduce the pressure in the
cuff to a value approximately equal to the initial pressure level P.sub.0
or below. Given the speed and ease of operation made possible by the
present invention, a repeat of the entire operation t.sub.0 -t.sub.4 may
be elected in which case the initial inflation of the cuff can be
immediately begun for the subsequent operation. A substantially
instantaneous pressure drop is indicated at time t.sub.4. Typically, final
depressurization occurs over time with a sloped or curved operating
characteristic. In a subsequent immediately consecutive operating
sequence, the systolic, MAP and diastolic values that were previously
observed can be used to adjust the beginning of the deflation period so as
to reduce the overall reading times.
An example of initial parameter determination will now be given.
The following is an example of the cuff detection process according to the
present invention, assuming a simplified system having five different cuff
sizes. Reference is made to the following table showing pressure-time
relationships empirically determined for each particular pneumatic control
system of interest. The pressure thresholds in the following table are
determined from pressure versus time curves for each cuff size. In the
following table, three initial pressure ranges and four cuff size
thresholds are considered.
Sample Table Of Cuff Determination Pressure Thresholds
Initial Pressure Pressure Pressure
Pressure Threshold Threshold Threshold
(mmHg) @ 11 @ 12 @ 13
0-4 58 74 80
4-7 80 119 130
7-10 94 132 151
Cuff Determination Process
1. The valve is open at the beginning of a blood pressure determination
cycle and the pressure in the cuff is monitored. If the pressure in the
cuff is >10 mmHg, the valve is left open until the pressure drops below 10
mmHg.
2. The initial pressure in the cuff is determined and the pressure
thresholds are obtained from the corresponding row of the table above
(i.e., if the initial pressure is 0-4 mmHg, the pressure thresholds at 11,
12 and 13 will be 58, 74 and 80 mmHg, respectively.
3. The valve is closed, and the pump is turned on at a fixed slow, flow
rate. The cuff pressure/time relationship (dP/dt) is measured and is
compared to the threshold for the smallest cuff first and then for
increasingly larger cuffs until a cuff size is determined.
4. The pressure in the cuff is monitored for up to 11 seconds while
inflating. If the pressure in the cuff exceeds the pressure threshold for
11 (58 mmHg in this example) prior to 11, the smallest cuff size has been
detected. The initial target inflation pressure, inflation, parameters,
and deflation parameters are then set for this cuff.
5. If the pressure threshold for 11 has not been exceeded, the pressure in
the cuff is monitored until either 12 seconds has been reached, or the
pressure threshold for 12 has been exceeded. If the pressure threshold for
12 has been exceeded prior to 12, the next larger cuff size has been
detected and he parameters are set for this cuff size.
6. If the pressure threshold for 12 has not been exceeded, step 5 is
repeated while monitoring for subsequent pressure and time thresholds
until a cuff size has been determined.
7. Once a cuff size has been determined, the inflation parameters are set
and the cuff is inflated until the initial target inflation pressure has
been reached. At this time the blood pressure reading cycle begins and the
cuff is deflated using the cuff deflation parameters for the detected cuff
size until a blood pressure determination is made.
The following parameter table was developed for the five cuff sizes
studied.
Sample Cuff Size Parameter Table
Initial
Target Inflation Inflation Deflation Deflation
Cuff Inflation Duty Overshoot Duty Overshoot
Size Pressure Cycle Parameter Cycle Parameter
1 100 30% -3 30% +4
2 125 40% -1 40% +1
3 148 70% 0 50% 0
4 170 100% +1 70% 0
5 170 100% +2 70% -1
The drawings and the foregoing descriptions are not intended to represent
the only forms of the invention in regard to the details of its
construction and manner of operation. Changes in form and in the
proportion of parts, as well as the substitution of equivalents, are
contemplated as circumstances may suggest or render expedient; and
although specific terms have been employed, they are intended in a generic
and descriptive sense only and not for the purposes of limitation, the
scope of the invention being delineated by the following claims.
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